U.S. patent number 6,057,538 [Application Number 08/923,442] was granted by the patent office on 2000-05-02 for image sensor in which each lens element is associated with a plurality of pixels.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to John A. Clarke.
United States Patent |
6,057,538 |
Clarke |
May 2, 2000 |
Image sensor in which each lens element is associated with a
plurality of pixels
Abstract
An image sensor 10 includes an array of light responsive pixels
22 and an array 30 of focusing lens elements 32. Each lens element
32 is associated with a plurality of pixels 22, so that the lens
elements may be fabricated simply. The array 30 is preferably part
of a non-inverting focusing arrangement, and the lens elements 32
are preferably refractive microlens elements.
Inventors: |
Clarke; John A. (Carshalton,
GB) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
10799590 |
Appl.
No.: |
08/923,442 |
Filed: |
September 4, 1997 |
Foreign Application Priority Data
Current U.S.
Class: |
250/208.1;
250/216; 257/E31.128 |
Current CPC
Class: |
H01L
27/14623 (20130101); H01L 31/02327 (20130101); H01L
31/02325 (20130101); H01L 27/14627 (20130101) |
Current International
Class: |
H01L
31/0232 (20060101); H01J 040/14 () |
Field of
Search: |
;250/208.1,216,239,214.1
;257/80-84,430-443,436 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Le; Que T.
Attorney, Agent or Firm: Biren; Steven R.
Claims
I claim:
1. An image sensor comprising an array of light-responsive pixels,
and an optical focusing arrangement comprising an array of focusing
lens elements associated with the array of pixels, wherein a pitch
of said lens elements is at least about twice a pitch of said
pixels and the array of focusing lens elements is arranged with
respect to the array of pixels such that each lens element is
optically associated with a respective plurality of pixels, whereby
light passing through each said lens element is provided to, and
focused on, each of its said respective associated plurality of
pixels.
2. An image sensor as claimed in claim 1, wherein the lens elements
are spaced apart on a common substrate and opaque regions are
provided between the lens elements.
3. An image sensor as claimed in claim 1, further comprising means
for blocking the passage of light to pixels associated with one
lens element from another lens element.
4. An image sensor as claimed in claim 3, wherein the means for
blocking comprises an array of light blocking elements arranged
substantially adjacent the array of pixels, the light blocking
elements being arranged over the spacing between pixels associated
with adjacent lens elements.
5. An image sensor as claimed in claim 4, wherein the light
blocking elements are arranged over the spacing between all
pixels.
6. An image sensor as claimed in claim 3, wherein the array of
pixels includes an array of light shielding portions disposed over
the light responsive pixels, each portion being associated with an
individual pixel and including a light receiving aperture.
7. An image sensor as claimed in claim 6, wherein the means for
blocking comprises an array of light blocking elements arranged
substantially adjacent the array of pixels, the light blocking
elements being arranged over the light shielding portions.
8. An image sensor as claimed in claim 1, further comprising at
least one further array of lens elements, the lens elements of the
lens array each having the same optical axis as a corresponding
lens element of the at least one further lens array, the lens array
and the at least one further lens array together comprising a
non-inverting optical system.
9. An image sensor as claimed in claim 8, wherein the non-inverting
optical system comprises three arrays of lens elements.
10. An image sensor as claimed in claim 1, wherein the lens
elements comprise substantially circular converging lenses having a
diameter between 0.1 mm and 3 mm.
Description
BACKGROUND OF THE INVENTION
This invention relates to an image sensor comprising an array of
light responsive pixels. The invention is particularly concerned
with focusing of an image on to the array of pixels for a
contact-type image sensor.
For a focused image to be formed on a pixel array of a contact-type
image sensor, it is possible either to position the original
document at a very short distance from the array of pixels, or to
provide an optical system for focusing.
If no optical system is provided, the document to be imaged should
preferably be spaced from the array of pixels by a distance which
is no greater than the pitch of the pixels, for example. If there
is greater spacing of the document from the array of pixels, each
pixel may receive light from a greater area of the document to be
imaged than is desired. As the resolution of an image sensor is
increased, the image sensing pixels are arranged with progressively
smaller pitch, so that for high resolution image sensors it is not
possible to maintain sufficiently small spacing between the
document and the array of pixels, and the need arises for an
optical focusing arrangement.
U.S. Pat. No. 5,286,605 discloses a solid-state imaging device
having a microlens array provided over the image sensor array, each
microlens element being associated with an individual pixel of the
imaging device. Of course, small microlens are required which must
be accurately aligned with the pixels of the image sensor.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an image
sensor comprising an array of light responsive pixels, and an array
of focusing lens elements associated with the array of pixels, each
lens element being associated with a respective plurality of
pixels.
The use of lens elements associated with a group of pixels enables
an increase in the size of each lens element. The size of the lens
elements may be selected such that simplified fabrication
techniques may be employed for manufacturing the lens array.
The lens elements are preferably spaced apart on a common
substrate, and opaque regions are then provided between the lens
elements. The opaque regions ensure that all light received by the
array of pixels has been focused by the lens elements.
Means may be provided for blocking the passage of light to pixels
associated with one lens element from another lens element. These
prevent the appearance of so-called "ghost images".
The blocking means may comprise an array of light blocking elements
arranged substantially adjacent the array of pixels, the light
blocking elements being arranged over the spacing between
pixels.
The array of pixels preferably include an array of light shielding
portions disposed over the light responsive pixels, each portion
being associated with an individual pixel and including a light
receiving aperture. This enables an effective reduction in the size
of each pixel (which reduces the range of light detected) whilst
maintaining a high pixel capacitance for image storage. In this
case, the blocking means preferably comprises an array of light
blocking elements arranged substantially adjacent the array of
pixels, the light blocking elements being arranged over the light
shielding portions.
The image sensor may comprise at least one further array of lens
elements, the lens elements of the lens array each having the same
optical axis as a corresponding lens element of the at least one
further lens array, the lens array and the at least one further
lens array together comprising a non-inverting optical system.
The use of a non-inverting optical system avoids the need for image
processing which is required if the lens elements produce local
inversion of portions of the image to be sensed. Furthermore, the
use of a non-inverting system avoids the need for accurate
alignment of the lens elements with the associated pixels. The
non-inverting optical system preferably comprises three arrays of
lens elements.
The lens elements may comprise microlenses, for example refractive
or holographic lenses, or the lens array may comprise a planar
array of graded index lenses.
The lens elements may be substantially circular converging lenses,
each having a diameter between 0.1 mm and 3 mm. The resolution of
the image sensor will then determine the number of pixels
associated with each lens element.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described by way of example, with
reference and as shown in the accompanying drawings in which:
FIG. 1 shows an image sensor according to the invention having an
optical focusing arrangement;
FIG. 2 shows an image sensor according to the invention with a
first alternative optical focusing arrangement;
FIG. 3 shows an image sensor with a second alternative optical
arrangement.
FIG. 4 shows an image sensor with a third alternative optical
focusing arrangement;
FIG. 5 shows an image sensor according to the invention with an
optical focusing arrangement corresponding to that of FIG. 3, the
image sensor further comprising a light blocking arrangement;
FIG. 6 shows an image sensor according to the invention with an
optical focusing arrangement corresponding to that of FIG. 1, the
image sensor further comprising a first alternative light blocking
arrangement; and
FIG. 7 shows a second alternative light blocking arrangement.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an image sensor 10 comprising a substrate 20 which
carries a plurality of image sensing pixels 22 and an optical
focusing arrangement 30 which provides a focused image of an
original 40 on to the pixels 22. In accordance with the invention,
the optical focusing arrangement 30 comprises a plurality of lens
elements 32 (three of which 32A, 32B, 32C are shown in FIG. 1) each
associated with a respective group of pixels 22.
Contact-type image sensors are known which employ an optical
focusing arrangement in the form of an array of microlens elements
formed over the image sensing pixels, and with an individual
microlens element associated with each image sensing pixel. This
arrangement requires each microlens to occupy an area which is no
greater than the area associated with an individual pixel. The
reduction in pixel size required for high resolution imaging
applications gives rise to very small pixel dimensions, and the
consequent need for very small microlenses. For example, image
sensing applications may require a resolution of 600 dpi (dots per
inch) which gives rise to a pixel pitch of 42 .mu.m (micrometres).
Known methods exist for forming shaped lens elements of diameter as
low as 50 .mu.im from reformed thermoplastic resin. For this
purpose, a substrate of the lens array (usually glass) is coated
with a thermoplastic resin layer, which may be applied by means of
spin coating. Subsequent patterning of the thermoplastic resin may
be carried out by means of photolithography. This gives a
thermoplastic resin layer having discrete portions, each
corresponding to an individual microlens, in the desired positions.
Thermal reflow of the resin layer at a predetermined temperature
causes the thermoplastic resin to reform in a convex lens shape.
However, as the size of the microlens elements is reduced, it
becomes increasingly difficult to control the power of each lens
element and the cost of producing the lens array increases.
By associating a group of pixels 22 with each lens element 32
according to the present invention, it becomes possible to increase
the size of each lens element to enable less expensive fabrication
techniques. The lens elements may, for example, be formed by known
moulding techniques. Thus, a mould may be formed by creating
indentations in a metal plate and this mould may be used to make
plastic retractive lenses on a glass substrate
or a plastic sheet.
The image sensing pixels 22 shown in FIG. 1 are arranged in a two
dimensional array. The array may be of known configuration and may,
for example, comprise thin film semiconductor layers deposited in
appropriate patterns on the substrate 20 to define the array of
pixels 22. Each pixel preferably includes a photodiode, and various
photodiode pixel arrangements will be known to those skilled in the
art. Alternatively, the array of image sensing pixels 22 may
instead form part of an electrostatic imaging system, or the pixels
22 may comprise charge coupled devices.
Each image sensing pixel 22 may include an array of light shielding
portions disposed over the pixels, each portion being associated
with an individual pixel and having a sensor aperture 23 enabling
light to pass to a portion of the pixel 22. In this way, a small
light receptive area of the sensor is obtained while maintaining
the large capacitance needed to store the signal. The use of a
small light receptive area restricts the spread of angles of
incidence over which the pixels receive light, which assists in
reducing the possibility of light from one imaging area being
focused on to two or more adjacent image sensing pixels.
The focusing arrangement 30 represented in FIG. 1 comprises a
single array of microlens elements 32 which, as described above,
may be formed by conventional moulding techniques. Instead,
holographic lenses may be employed or the array of microlens
elements may comprise a planar array of graded index lenses. In the
representation of FIG. 1, each microlens element 32 is shown to be
associated with three pixels 22A, 22B, 22C. In two dimensions, each
microlens element is therefore associated with a group of nine
pixels, arranged in a 3.times.3 sub-array. In practice, each
microlens element 32 may be associated with a much greater number
of image sensor pixels, for example of the order of 100 or more, so
that the fabrication of the microlens array becomes less
complicated. A typical pitch of 0.1 mm to 3 mm is provided for the
microlens elements, and the resolution of the image sensor 10 will
then determine the number of image sensing pixels associated with
an individual microlens element 32.
In the example of FIG. 1, each lens element 32 forms an inverted
image of a portion of the original 40. This is illustrated in FIG.
1 using light envelopes associated with each pixel 22. When each
microlens element is associated with an individual pixel, as is
conventional, the local inversion caused by the individual
microlens elements 32 does not distort the image formed on the
array of pixels 22. However, when each microlens element 32 is
associated with a group of pixels, then discontinuities occur at
the boundary between one group of pixels (associated with one lens
element) and the next group (associated with an adjacent lens
element). The use of the optical focusing arrangement 30 shown in
FIG. 1 therefore requires processing of the signals received by the
pixels in order to reconstruct an image of the original 40.
Although such image processing is possible, the use of the optical
system 30 of FIG. 1 also requires a knowledge of exactly which
pixels are associated with each lens element 32, so that the
correct signal processing can take place. When each lens element 32
is associated with a large number of very small pixels, the
accuracy with which the array of lens elements 32 must be
positioned is undesirably great.
The above disadvantages of the system shown in FIG. 1 may be
overcome by using a non-inverting optical focusing arrangement, as
will be described in further embodiments.
Irrespectively of the exact configuration of the optical focusing
arrangement 30, the arrangement 30 provides a depth of focus 42
within which the original 40 can be positioned so as to be focused
on the image sensing pixels 22. This depth of focus 42 preferably
has a sufficient range to enable local deformation of the original
40 without the image becoming out of focus, and is also preferably
spaced from the optical focusing arrangement 30 by a sufficient
distance to enable an imaging window to be positioned over the
focusing arrangement 30 so as to provide a surface on which the
original 40 is positioned. Various spacing layers may be
appropriate to ensure correct relative positioning of all
components of the image sensor 10.
The focusing arrangements 30 has opaque portions 34 disposed
between the lens elements 32 which ensure that all light received
from the original 40 passes through the lens elements 32 and can
not pass directly through the substrate 33 of the lens array. The
lens elements 32 may alternatively abut one another, for example
forming a honeycomb structure. However, circular lens elements may
be preferred, in which case the spacing between the lens elements
32 is provided with blocking material 34, for example by a printing
process. It may also be desirable to employ lens elements 32 which
are smaller than the maximum possible size if a shorter focal
distance is desired, so as to reduce the overall thickness of the
image sensor 10.
In addition to the light blocking material 34, it is also desirable
to employ measures for eliminating so-called "ghost images" which
result from light falling on pixels 22 from lens elements 32
associated with a different group of pixels. For example, in FIG. 1
arrow 35 represents the possible passage of light to pixel 22A
(which is associated with lens element 32B) but which has passed
through lens element 32A. Various measures may be employed for
eliminating ghost images, in the form of light blocking material
and/or arrays of apertures. Some examples of the possibilities
available are described below.
As described above, it may be preferred to employ a non-inverting
optical focusing arrangement 30 and one possibility is represented
in FIG. 2. The same reference numerals have been used to denote
similar parts to FIG. 1. For the purposes of a clear
representation, each lens element of the focusing arrangement 30 is
again shown to be associated with a line of three pixels 22 (and
therefore a sub-array of nine pixels).
In FIG. 2, the optical focusing arrangement 30 comprises three lens
arrays 50, 52, 54 which together form a non-inverted image of the
original 40. An inverted image is formed by one array 50, and this
array is re-imaged by a subsequent array to form a non-inverted
image. The purpose of the central lens array is described below.
For any particular spacing of the lens arrays, there is an
associated required position of the original and of the array of
pixels for focusing to take place. This will be evident to those
skilled in the art. As a result, the spacing of the original 40
from the focusing arrangement 30, the spacing between the lens
arrays 50, 52, 54, and the spacing of the array of sensing pixels
22 from the focusing arrangement 30 are each selected in
combination to arrive at a practical implementation of the
invention.
During operation of the image sensor shown in FIG. 2, each lens
element 51 closest to the original receives light from a number of
regions of the original. The lens array 50 of lens elements 51
forms an inverted image of these regions close to lens array 52.
Each lens element 55 in the lens array 54 then forms a non-inverted
image on the array of image sensing pixels 22. The purpose of the
lens array 52 is to prevent light spreading. If the lens array 52
were omitted, light from the lens elements 51 could reach several
of the lens elements 55. Other lens arrangements will be apparent
to those skilled in the art.
In FIG. 2, the lens arrays 50, 52, 54 are shown on separate
substrates. It will be apparent that the arrays 50 and 52 could be
formed on opposite sides of a single substrate of appropriate
thickness. Alternatively, by reversing array 52, arrays 52 and 54
could be formed on a single substrate.
Yet another alternative is for the power in the lens array 52 to be
divided between two arrays, each of which is then formed on the
other side of either array 50 or array 54, as shown in FIG. 3.
In both FIGS. 2 and 3, light blocking material 34 is represented
between the lens elements of each lens array 50, 52, 54, and having
the same function as the blocking material 34 explained with
reference to FIG. 1.
A further alternative for the optical focusing arrangement 30 is
shown in FIG. 4, and uses an additional microlens array 60 between
the original and the remainder of the focusing arrangement 50, 52,
54. The purpose of the additional lens array 60 is to bend the rays
travelling to off-axis pixels from regions of the original, so that
they are closely parallel to the optical axis when near the
original 40. If the original 40 is then slightly out of focus, the
error in its apparent position will be minimized, thereby
effectively increasing the depth of focus.
As described above, additional measures may be desired to prevent
the formation of so-called "ghost images", by blocking the passage
of light represented by arrow 35 in FIG. 1. Of course, the passage
of light from a lens element 32 to the associated pixels 22 must
not be prevented, and FIG. 5 part A shows one possible arrangement
of light blocking material 62 forming ghost image attenuators. As
shown, the light blocking material 62 is provided over the image
sensing pixels 22 between the apertures 23. The light blocking
material 62 is arranged as a plurality of stacks which taper away
from the image sensing pixels, so that they do not interfere with
the passage of light from the lens elements to the associated
pixels. The exact form of the stacks of light blocking material and
their positions will depend upon the optical arrangement. The
stacks may, for example, be arranged as a regular array with the
stacks offset (in both orthogonal axes within the pixel array) from
the pixels 22 by half of the pixel pitch. This is shown in FIG. 5
part B. Each stack is shown as a truncated square-based
pyramid.
A modification to the arrangement of FIG. 5 is shown in FIG. 6 and
may be employed when illumination of the document to be imaged is
carried out through the substrate 20 of the image sensor 10.
In FIG. 6, the light blocking material 62 is arranged over each
image sensing pixel 22 in pyramids disposed over the blocking
portions of the sensor apertures 23. In this way, the light
blocking material 62 does not prevent the passage of light from
beneath the substrate 20 to the optical focusing arrangement 30
through the spaces between sensor pixels 22.
A further alternative is shown in FIG. 7, in which a lattice of
light blocking walls 64 surrounds each group of pixels, the walls
being disposed over the spacing between pixels of different groups,
the lattice blocking the passage of light from adjacent lens
elements. The configuration of walls 64 will depend upon the shape
of the pixel groups, and may, for example, form a hexagonal
honeycomb structure, or a square or rectangular lattice.
From reading the present disclosure, other modifications will be
apparent to persons skilled in the art. Such modifications may
involve other features which are already known in the design and
use of electrical or electronic circuits and component parts
thereof and which may be used instead of or in addition to features
already described herein. Although claims have been formulated in
this application to particular combinations of features, it should
be understood that the scope of the disclosure of the present
application also includes any novel feature or any novel
combination of features disclosed herein either explicitly or
implicitly or any generalisation of one or more of those features
which would be obvious to persons skilled in the art, whether or
not it relates to the same invention as presently claimed in any
claim and whether or not it mitigates any or all of the same
technical problems as does the present invention. The applicants
hereby give notice that new claims may be formulated to such
features and/or combinations of such features during the
prosecution of the present application or of any further
application derived therefrom.
* * * * *